Combination muffler and spark arrester

ABSTRACT

An improved combination muffler and spark arrester for an exhaust system of an internal combustion engine includes a muffler section, a diverging inlet pipe section that defines, optionally in combination with the muffler section, an enlarged volume, and a spark arrester screen extending across the entire flow path to the muffler section. This combination provides effective spark arresting without impeding exhaust flow, and therefore without any significant adverse effect on performance.

FIELD OF THE INVENTION

This invention relates to exhaust systems for motor vehicles, and more particularly to an exhaust system employing a sound attenuating device in combination with a spark arrester to achieve improved power output and sound reduction.

BACKGROUND OF THE INVENTION

The exhaust system of an internal combustion engine is typically designed to perform a plurality of functions. A primary function of the exhaust system is to convey hot exhaust gases away from the engine and discharge the exhaust gases to atmosphere at a location away from the operator to minimize driver and passenger exposure to the exhaust gases. Another important function of the exhaust system is to silence or muffle hazardous and objectionable noises. In general, exhaust systems have achieved improved sound attenuation at the expense of reduced performance, i.e., lower power output.

During the operation of motorized vehicles in certain environments, such as in forests or other wooded areas that are susceptible to fire, it is desirable, and often mandatory, that the exhaust system include a spark arrester that prevents hot particles or sparks from passing through the exhaust system and into the atmosphere where they could ignite combustible material and cause substantial damage. This a particular concern with smaller vehicles, such as snowmobiles and so called “all terrain vehicles,” which have an exhaust system of relatively short length as compared with a typical automobile. Conventionally, screens have been incorporated in such exhaust systems to block the passage of hot particles. However, screens of this type have typically been troublesome and have required frequent cleaning to avoid serious reduction in the efficiency of the engine.

Therefore, it would be desirable to provide an exhaust system employing an improved combination sound attenuating device and spark arrester that has a reduced tendency for plugging, and does not adversely affect power output.

U.S. Pat. No. 3,757,892 to Raudman, Jr. discloses an exhaust unit for a combustion engine. The exhaust unit includes a muffler in a cylindrical housing, a truncated conical section that provides a transition to the major cylindrical portion of the housing, and a somewhat conical or concave-convex screen spaced apart from and matingly aligned with the conical section of the housing to provide a somewhat conical, annular passage. The screen functions to pass a major portion of the exhaust stream directly to a muffler section, while deflecting a fragment of the stream, along with particles greater than the screen mesh, and causing the deflected stream to flow radially through the passage and into an annular space in which particles are broken down to a safe size and passed to the muffler section for release from the unit.

U.S. Pat. No. 3,884,655 to Coop discloses a two-section spark arrester and muffler for an internal combustion engine that includes a core which has a tube open at one end for the outlet of gases and is provided with longitudinally and annularly spaced openings covered by screens. The opposite end of the tube is closed against entry of gases. Longitudinally spaced cylindrical conduits, each having an open end, are concentrically disposed around the tube and screen openings. The opposite ends of the conduits are connected to the tube and a conical deflector is connected to the end of the conduit covering the openings in the closed end of the tube. A shell concentrically disposed around the conduits has an inlet opening adjacent the deflector and is connected to the tube at its open end to provide co-acting muffler sections for the exhaust gas.

SUMMARY OF THE INVENTION

The invention provides an improved exhaust system for an internal combustion engine that utilizes an enlarged space disposed upstream of a muffler, and a spark arrester screen disposed at least partially within the enlargement. The arrangement has several advantages. A primary advantage is that by disposing the flame arrester screen in an enlargement upstream of the muffler it is possible to increase the screen surface area to allow effective spark arresting without impeding flow of the exhaust gases. As a result, there is not any, or very little, adverse affect on engine performance caused by the spark arrestor. A second advantage is that the arrangement makes it possible to take advantage of a negative pressure wave that is produced as sound waves traverse the enlargement and generate a partial vacuum in their wake. The partial vacuum (i.e., a pressure below ambient atmospheric pressure) helps pull exhaust gases through the spark arrester screen.

In accordance with one aspect of the invention, the exhaust system includes a muffler, a conduit for conveying combustion gases from an exhaust port of an internal combustion engine toward the muffler, a diverging pipe section defining an enlarged volume disposed between the conduit and the muffler, and a spark arrestor screen disposed completely within the enlarged volume.

In accordance with another aspect of the invention, an exhaust system is provided which includes a muffler, a conduit for conveying combustion gases from an exhaust port of an internal combustion engine toward the muffler, a diverging pipe section defining an enlarged volume disposed between the conduit and the muffler, and a spark arrester screen at least partially disposed within the enlargement, the spark arrester screen extending across the entire flow path to the muffler.

In accordance with a further aspect of the invention, there is provided a combination muffler and spark arrester for the exhaust system of an internal combustion engine, which includes an imperforate outer shell, a perforated intermediate shell spaced inwardly of the outer shell, sound absorbing material disposed within a first volume defined between the outer shell and the intermediate shell, a perforated inner shell spaced inwardly of the intermediate shell, sound absorbing material disposed within a second volume defined with the inner shell, the inner shell and the intermediate shell defining an annular space for flow of exhaust gases from an internal combustion engine, the annular space having an inlet end and an outlet end, a diverging inlet pipe section defining an enlarged volume to an inlet end of the annular space, and a spark arrester screen at least partially disposed within the enlarged volume.

These and other features, advantages and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims and appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal cross section of a combination muffler and spark arrester for the exhaust system of an internal combustion engine in accordance with a preferred embodiment of the invention.

FIG. 2 is a longitudinal cross section of a combination muffler and spark arrester for the exhaust system of an internal combustion engine in accordance with a preferred alternative embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention is directed to exhaust systems for an internal combustion engine which incorporate a muffler and a spark arrester screen which is located in an enlarged pipe section upstream of the muffler. By positioning the spark arrester in an enlarged pipe section upstream of the muffler, it is possible to provide a spark arrester screen having a large area. This minimizes the extent to which the spark arrester screen presents an obstruction to the flow of exhaust gases through the exhaust system. As a result, the arrangement facilitates effective spark arresting without impeding exhaust flow, and thereby minimizes or eliminates any adverse affect on performance (power output) associated with use of the spark arrester screen. In fact, dynamometer testing has shown that power output is substantially identical with and without the spark arrester screen.

Further, by locating the spark arrester screen in an inlet diverging pipe section to the muffler it is possible to take advantage of a negative pressure wave that is produced as sound waves traverse the inlet diverging pipe section and produce a partial vacuum in their wake. The negative pressure wave (below ambient pressure) aids in flow through/pull through in the area of the spark arrester. This means that the negative wave assists propagation of exhaust gases through the spark arrester screen.

In FIG. 1 there is shown an exhaust system 10 including a muffler section 12, a diffuser section 14 for conveying exhaust gases from a head pipe 16 to the muffler section 12, and an outlet pipe 18 for conveying exhaust gases from the muffler section 12 to the remaining portion or portions of the exhaust system (not shown), which may include other tailpipe sections. The reference to a muffler section 12, diffuser inlet pipe section 14 and an outlet pipe 18 are meant to define functional sections of the exhaust system, and do not necessarily imply separate structural components.

The muffler or silencer section 12 of the exhaust system includes an outer shell 20 having imperforate walls. An intermediate shell 22 having perforated walls defined by perforations 24 is spaced inwardly of the outer shell walls 20. Tapered flow diverters or megaphone sections (discussed later) provide negative pressure pulses toward the engine.

A sound absorbing material 26 is disposed in a first volume defined by and disposed between the outer shell 20 and the intermediate shell 22. An inner shell 28 having perforated walls defined by perforations 30 is spaced inwardly of intermediate shell 22. Inner shell 28 defines a second volume having sound absorbing material 32 disposed therein. Defined by and disposed between intermediate shell 22 and inner shell 28 is an annular space 34 through which exhaust gases may flow. The direction of gas flow is indicated by flow arrows in FIGS. 1 and 2.

Outer shell 20, intermediate shell 22 and inner shell 28 are preferably concentric, and may have a circular, elliptical or oval transverse cross-sectional shape. Annular space 34 includes an inlet end 36 and an outlet end 38, and preferably has a constant cross-sectional area. Typically, the cross-sectional area available for flow through annular space 34 is greater than the cross-sectional area available for flow through head pipe 16.

Inlet pipe or diffuser section 14 has a variable inner diameter that increases in the direction of flow of the exhaust gases. This means that the diameter increases from the head pipe 16 to the inlet end 36 of the annular space 34. In the illustrated embodiment, inlet pipe 14 is a frustoconical pipe section having a minimum diameter about equal to the diameter of head pipe 16, and a maximum diameter about equal to the diameter of intermediate shell 22.

Projecting from the inner shell 28 away from inlet 36 of the annual space 34 is a tapered flow diverter 40. Flow diverter 40 has a pointed end or leading tip 42 and diverting imperforate walls 44 that guide exhaust gases from head pipe 16 to annular space 34. The flow diverter 40 may have a conical shape or a parabolic shape, with a base 46 having a transverse cross section that conforms with the transverse cross-sectional shape and dimensions of inner shell 28.

Outlet pipe 18 has an inner wall 48 having a variable inner diameter that increases in the directional flow of the exhaust gases. This means that the diameter increases from the outlet end 38 of annular space 34 toward the tail end of the exhaust system.

In the embodiment shown in FIG. 1, an outlet diverter 50 is provided. Outlet diverter 50 may have a conical or parabolical longitudinal cross section. Outlet diverter 50 includes a trailing tip 52 and a leading base 54 having a transverse cross-sectional shape and size about equal to the transverse cross-sectional shape and size of inner shell 28. Outlet diverter 50 includes walls that converge from outlet end 38 of inner shell 28 defining annular flow volume 34 toward trailing tip 52 to provide an expanded flow path.

In a modern four-stroke engine, there is a brief period between the end of an exhaust stroke and the beginning of an intake stroke when the camshaft actually has both the intake exhaust valves open simultaneously. This period of cam timing is known as “overlap.” The exhaust system intent of this invention is able to produce a negative pressure wave at the exhaust valve during overlap. Because intake and exhaust valves are at opposite ends of the combustion chamber, a negative pressure at the exhaust valve during overlap causes a sweeping flow of a mixture of fresh fuel and air from the intake valve to the exhaust valve and effectively removes what would be the remnant portion of the exhaust gas that would dilute the fresh mixture charge. This effect is referred to as “scavenging,” because the combustion chamber becomes scavenged or swept clean of combustion products. This can also create a condition of lower than atmospheric pressure in the combustion chamber before the piston begins its downward intake stroke. This subatmospheric pressure or partial vacuum assists in initiating flow into the cylinder from the intake valve.

The synchronization of negative pressure and overlap timing is dependent on rotational speed (RPM). The best way to achieve suction (partial vacuum) and overlap harmony is by providing an exhaust that creates suction over a long period of time, and which exhibits inherent tunability to allow time shifting of the suction events to match changes in engine tuning and RPM range demands of different operating environments.

In a properly designed exhaust system, the propagation of exhaust flow and of sound pressure wave flow are taken into consideration. Exhaust flow can be impeded by sharp bends, reduced pipe diameters and non-aerodynamic obstacles. Exhaust flow includes particulate matter, has mass, and behaves as a fluid. Thus, anything that would normally disrupt a fluid flow would also slow exhaust flow. The sound pressure wave flow is unaffected by sharp bends, reduced diameters, etc. However, the sound pressure wave flow has properties that may be exploited in the design of an exhaust system. When a confined pressure wave encounters an enlargement in its containment area a negative pressure wave is sent back toward the origin of the pressure wave. Conversely, a reduction in space reflects back a positive pressure wave. Thus, an exhaust pulse that exits from a pipe into atmospheric pressure sends back, a negative pressure wave of very short duration that is proportional to the abruptness in the change of its confinement. An exhaust pulse that transitions from a pipe to open atmosphere via a diverging pipe section (i.e., a megaphone-like pipe) returns a negative pressure through the pipe of less intensity but of longer duration.

A megaphone exhaust is desirable for broad range power outputs since the longer duration negative pressure wave has a greater possibility of being synchronous with a given camshaft overlap period. However, a megaphone exhaust is usually very loud.

The disclosed muffler arrangement takes advantage of the desirable broad range power output of a megaphone exhaust while overcoming the undesirable loudness by utilizing two megaphones (i.e., an inlet pipe having an increasing cross-section area for flow, and an outlet pipe having a decreased cross-sectional area for flow) separated by a dual core annular flow silencing section 12. The split megaphone design facilitates and broadens the time of the negative pressure wave, approaching the effectiveness of a conventional megaphone design without the obnoxious sound output. Unimpeded flow of exhaust gases is aided by an aerodynamic/bullet-shaped flow diverter 40 at the inlet megaphone 14. The exhaust system utilizes a baffleless flow-through design that minimizes constrictions that would create undesirable negative pressure harmonics. The exhaust system is changeable and/or tunable to the extent that parabolic or conical diverters 40 and 50 may be used to lengthen and recover negative pressure effects for enhanced evacuation during overlap scavenging timing. The angles and lengths of the megaphone cones 14 and 18 may be changed in conjunction with diverters 40 and 50 to alter the intensity and duration of the negative pressure waves. Accordingly, the exhaust system may be provided as a kit having changeable diverters and megaphone cones to facilitate tunability for different engines and/or different performance objectives. Enhanced sound absorption is achieved by a utilizing a straight-through design having high surface area.

Sound absorbing materials 26 and 32 may be the same or different. Suitable sound absorbing materials include fibrous metal, glass, polyarimides; glass or ceramic open cell foams; ceramic wool or felt; multiple layers of fine screening; etc. Combinations of these and/or other sound absorbing materials may be used. Perforations 24 and 30 may be arranged in any suitable pattern and have a suitable diameter to optimize sound absorption. In general, it is desirable that the perforations are uniformly spaced apart on shell walls 22 and 28. Circular holes or perforations are preferred, and typically have diameters in the range of from about 0.050 to about 0.375 inches.

Disposed within the enlarged volume defined by diverging inlet pipe section 14 is spark arrester screen 200. In the illustrated embodiment, spark arrester screen 200 has a conical shape similar to inlet pipe section 14, and extends across the entire flow path to the muffler. The openings or mesh size of screen 200 is selected to temporarily block large particles or sparks from passing through the exhaust system and out into the atmosphere where such sparks could ignite a fire. Suitable wire screens that may be employed have a mesh size of, for example, 20 or 30 open spaces per linear inch. It is believed that there are very few large particles or sparks that are generated during internal combustion that could potentially ignite a fire in the absence of a flame arrester. Accordingly, the potential for accumulating large amounts of such particles on the inlet side of flame arrester screen 200 is relatively minimal, especially since pulsating action in the exhaust stream will eventually cause most of the larger partials to break down into smaller particles that can pass through screen 200.

An alternative embodiment 110 is shown in FIG. 2. Exhaust system 110 includes a muffler section 12 and an outlet pipe section 18 that are similar to those described with respect to the embodiment 10 shown in FIG. 1. However, the inlet pipe section 114 differs to the extent that the inner diameter of section 114 increases to a diameter greater than that of the outer diameter of the annular space 34. Specifically, the maximum inner diameter of section 114 is greater than the diameter of intermediate shell 22. In the illustrated embodiment, the inner diameter of inlet pipe 114 has a minimum diameter about equal to the diameter of head pipe 16, and a maximum diameter about equal to the diameter of outer shell 20. An annular or ring-shaped flow diverter 141 having a surface 143 is provided to smoothly guide (i.e., with a minimum of turbulence) exhaust gases from the enlarged chamber 145 defined by pipe 114 into the annular flow space 34. Flow diverter 141 is radially disposed between outer shell 20 and intermediate shell 22. Surface 143 may be curved in longitudinal cross section (as shown) or flat.

Disposed within an enlarged space defined by outer shell 20 and diverging pipe section 114 is a spark arrester screen 200. Spark arrester 200 has a conical shape with a leading pointed end. The other end terminates at the inlet end of intermediate shell 22, thereby completely extending across the entire flow path from head pipe 16 to muffler section 12 so that all materials flowing into the muffler section must pass through flame arrester screen 200. Flame arrester screen 200 is preferably mostly or entirely disposed within the enlarged volume defined by inlet pipe section 114 and optionally by a cylindrical extension of the muffler section (e.g., outer shell 20 or intermediate shell 22).

While the invention may be used with generally any exhaust system for an internal combustion engine, it is particularly useful for use with vehicles having relatively short exhaust systems that are used in forested areas or other areas where escaping sparks could cause a fire. Examples include snowmobiles, all terrain vehicles and off-road motorcycles.

The above description is considered that of the preferred embodiments only. Modifications of the invention will occur to those skilled in the art and to those who make or use the invention. Therefore, it is understood that the embodiments shown in the drawings and described above are merely for illustrative purposes and are not intended to limit the scope of the invention, which is defined by the following claims as interpreted according to the principles of patent law, including the doctrine of equivalents. 

1. A combination muffler and spark arrester for an exhaust systems of an internal combustion engine, comprising: a muffler section; a diverging inlet pipe section that defines, optionally in combination with the muffler section, an enlarged volume; and a spark arrester screen at least partially disposed within the enlarged volume, the spark arrester screen extending across the entire flow path to the muffler section.
 2. The combination muffler and spark arrester of claim 1, wherein the spark arrester screen has a conical shape.
 3. The combination muffler and spark arrester of claim 1, further comprising an inlet flow diverter disposed within the enlarged volume downstream of the spark arrester screen.
 4. The combination muffler and spark arrester of claim 1, wherein the spark arrester screen is completely disposed within the enlarged volume.
 5. The combination muffler and spark arrester of claim 1, wherein the muffler section includes an imperforate outer shell, a perforated intermediate shell spaced inwardly of the outer shell, sound absorbing material disposed within a first volume defined between the outer shell and the intermediate shell, a perforated inner shell spaced inwardly of the intermediate shell, sound absorbing material disposed within a second volume defined by the inner shell, the inner shell and the intermediate shell defining an annular space for flow of exhaust gases from an inlet end of the muffler section to an outlet end of the muffler section.
 6. A combination muffler and spark arrester for an exhaust system of an internal combustion engine, comprising: a muffler section; a diverging inlet pipe section that defines, optionally in combination with the muffler section, an enlarged volume; and a spark arrester screen disposed completely within the enlarged volume.
 7. The combination muffler and spark arrester of claim 6, wherein the spark arrester screen has a conical shape
 8. The combination muffler and spark arrester of claim 6, further comprising an inlet flow diverter disposed within the enlarged volume downstream of the spark arrester screen.
 9. The combination muffler and spark arrester of claim 6, wherein the spark arrester screen extends across the entire flow path to the muffler section.
 10. The combination muffler and spark arrester of claim 6, wherein the muffler section includes an imperforate outer shell, a perforated intermediate shell spaced inwardly of the outer shell, sound absorbing material disposed within a first volume defined between the outer shell and the intermediate shell, a perforated inner shell spaced inwardly of the intermediate shell, sound absorbing material disposed within a second volume defined by the inner shell, the inner shell and the intermediate shell defining an annular space for flow of exhaust gases from an inlet end of the muffler section to an outlet end of the muffler section.
 11. A combination muffler and spark arrester for an exhaust system of an internal combustion engine, comprising: a muffler section including an imperforate outer shell, a perforated intermediate shell spaced inwardly of the outer shell, sound absorbing material disposed within a first volume defined between the outer shell and the intermediate shell, a perforated inner shell spaced inwardly of the intermediate shell, sound absorbing material disposed within a second volume defined by the inner shell, the inner shell and the intermediate shell defining an annular space for flow of exhaust gases from an inlet end of the muffler section to an outlet end of the muffler section; a diverging inlet pipe section that defines, optionally in combination with the muffler section, an enlarged volume; and a spark arrester screen at least partially disposed within the enlarged volume.
 12. The combination muffler and spark arrester of claim 11, wherein the spark arrester screen has a conical shape.
 13. The combination muffler and spark arrester of claim 11, further comprising an inlet flow diverter disposed within the enlarged volume downstream of the spark arrester screen.
 14. The combination muffler and spark arrester of claim 11, wherein the spark arrester screen is completely disposed within the enlarged volume.
 15. The combination muffler and spark arrester of claim 11, wherein the spark arrester screen extends across the entire flow path to the muffler section. 